A Study of the String Effect in $Z \to 3$ Jets

The string effect is analyzed in events of the type Z -> qqbar with a hard gluon radiated. This effect is caused by color forces between the gluon and the two quarks in qqg-events, leading to increased final state particle densities between a gluon and a quark. Three-jet events are selected by applying a cluster algorithm to the charged and neutral particles measured with the ALEPH detector at the LEP collider. Exploiting the fact that, in QCD, jet 3 is the gluon jet with high probability if the jets are numbered according to their energy (E_1>E_2>E_3), two observables sensitive to the string effect are studied: the ratio R := N_{1,3}/N_{1,2} of inter-jet particle flow, with and without momentum weighting, and delta_phi, the angular shift between the leading particle and the total jet vector for jets 1 and 2. Using these observables the string effect is clearly seen in the data. Comparing the experimental results with predictions from various QCD Monte Carlo models, perturbative effects as well as different mechanisms in the hadronization process are identified, which are contributing to the string effect.